Field of the Invention
The present invention relates to a backlight unit, a liquid-crystal display apparatus, and a stacked structure, and specifically, relates to an edge-lit backlight unit in which light exiting from a primary light source, for example, enters a light guide plate from an edge (end surface) of the light guide plate and exits from an exit surface while propagating through the light guide plate and a liquid-crystal display apparatus which uses the edge-lit backlight unit.
Moreover, the present invention relates to a liquid-crystal display apparatus which improves the efficiency of utilizing light exiting from a primary light source using a reflective polarizing plate.
Related Art
Conventionally, in a liquid-crystal display apparatus, illumination light is supplied from an edge-lit backlight unit or the like to a liquid-crystal display panel to form a desired image. Moreover, in the edge-lit backlight unit, a primary light source is formed of a rod-shaped light source which uses a cold-cathode ray tube, a point-shaped light source which uses a light-emitting diode, and the like, and light exiting from the primary light source enters a light guide plate from an end surface (edge) of the light guide plate and propagates through the light guide plate. The light guide plate has countermeasures such as means for irregularly reflecting or diffusing the light which has been output from the primary light source and propagates through the light guide plate. Thus, the light guide plate gradually outputs light from an exit surface while allowing the light exiting from the primary light source to propagate through the light guide plate. In the edge-lit backlight unit, the light exiting from the light guide plate is supplied to a liquid-crystal display panel while correcting the directivity of the light using a prism sheet so that the light is directed in a front-surface direction of the exit surface. In this manner, in the edge-lit backlight unit, a surface light source provided for illumination of the liquid-crystal display panel is formed using the light exiting from the primary light source which is formed of a rod-shaped light source or a point-shaped light source.
With regard to such an edge-lit backlight unit, Patent Documents 1 and 2 propose a method of repeatedly forming a convex structure or the like having a pentagonal cross-sectional shape on an exit surface of a light guide plate to control the light exiting from the light guide plate so as to have sharp directivity in a front-surface direction of the exit surface to thereby improve the efficiency of utilizing the exiting light.
Moreover, Patent Document 3 discloses a structure in which a so-called reflective polarizing plate is disposed instead of a linear polarizing plate reflect the exiting light components of a backlight unit which have otherwise been absorbed by the linear polarizing plate so as to re-enter a light guide plate. According to this structure, it is possible to reutilize the re-entering output light and to improve the efficiency of utilizing the exiting light.
Further, Patent Document 4 discloses a configuration of a backlight unit in which a prism sheet having an upwardly convex shape is disposed on an exit surface of a light guide plate, and a ¼-wavelength plate and a reflective polarizing plate are sequentially disposed between the prism sheet and a liquid-crystal display panel. In this configuration, polarized light components reflected from the reflective polarizing plate are converted into circularly polarized light by the ¼-wavelength plate and re-enter the light guide plate, and the direction of the circularly polarized light is reversed when the circularly polarized light re-enters the light guide plate and is reflected inside the light guide plate. As a result, when the light exiting from the light guide plate re-enters the reflective polarizing plate, the light enters the reflective polarizing plate as the linearly polarized light that passes through the reflective polarizing plate. Due to this, in this configuration, the polarized light component reflected from the reflective polarizing plate is positively converted into polarized light components that pass through the reflective polarizing plate so as to further improve the light utilization efficiency.
However, the configuration disclosed in Patent Document 4 has a problem in that the configuration of the prism sheet becomes complex in order to supply the light exiting from the prism sheet to the liquid-crystal display panel with a sufficiently high peak light intensity and sharp directivity. Thus, such a configuration is not sufficient for practical use. That is, when the prism sheet is formed simply by repeatedly forming a convex structure having a triangular cross-sectional shape, desired directivity is secured by controlling an apex angle of the convex structure. However, in this case, it becomes difficult to suppress the sidelobe light sufficiently at an apex angle at which the peak light intensity is held at the largest value. As a result, it is necessary to focus on making changes on the cross-sectional shape of the convex structure and to take measures such as to form a light shielding portion in a partial region as disclosed in Patent Document 4.
Moreover, the display screen of a liquid-crystal display apparatus having the configuration disclosed in Patent Document 4 appears yellowish. Thus, such a configuration is not sufficient for practical use due to color tints of the display screen.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H08-254606
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-5505
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2000-227518
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2013-47794